The present invention relates to an apparatus useful in the production of hydrocarbon gases and oils from hydrocarbonaceous solids by the thermal pyrolysis of the solids. Specifically, the present invention is an apparatus for the removal of spent solids, ash and fine materials from an upflow bed of particulate oil shale rock undergoing thermal pyrolysis in a retort.
Vast deposits of oil shale, a sedimentary inorganic rock containing about 35 weight-percent calcite (CaCO.sub.3), 15 weight-percent dolomite (MgCO.sub.3 .multidot.CaCO.sub.3), and 10 weight percent alkali metal salts are known to exist in the United States, especially in the Green River formation in Colorado, Utah, and Wyoming. The oil shale in these deposits contains between 5 and 35 weight-percent of hydrocarbons in a form known as kerogen. When pyrolized, this kerogen decomposes to produce crude shale oil vapors, which, upon condensation, become a valuable source of fuel.
Several pyrolytic processes have been developed to produce crude shale oil from oil shale rock, e.g., those disclosed in U.S. Pat. Nos. 3,361,644; 4,069,132; 4,162,960; and 4,243,510, all of which are incorporated herein by reference. In a typical process, particulate oil shale rock is fed upwardly through a vertical retort by means of a reciprocating piston. The upwardly moving oil shale rock continuously exchanges heat with a downwardly flowing high-specific-heat, hydrocarbonaceous recycle gas introduced into the top of the retort at about 1200.degree. F. In the upper section of the retort (the pyrolysis zone), the hot recycle gas educes hydrogen and hydrocarbonaceous vapors from the oil shale rock. In the lower section (the preheating zone), the oil shale rock is preheated to pyrolysis temperatures by exchanging heat with the mixture of recycle gas and educed hydrocarbonaceous vapors plus hydrogen. That portion of the retorting apparatus wherein this heat exchange and pyrolysis takes place is generally referred to as a kiln. Most of the heavier hydrocarbons condense in the lower section and are collected at the bottom of the retort as a product oil. The uncondensed gas is then passed through external condensing or demisting means to obtain more product oil. The remaining gases are then recovered as a product gas, some of which is used as the recycle gas discussed previously, and some is combusted to heat the recycle gas to the hereinbefore specified temperature of 1200.degree. F., with the remainder being a source of fuel for any other purpose.
As the oil shale moves upward, it eventually reaches the top of the kiln and the retorted material, e.g. spent solids, ash and fine materials spill over onto a chute leading to a combustor, a steam stripping chamber, or a sealing chamber leading to an ash disposal area. Although gravity causes some of the retorted material to spill onto the chute, the amount that spills onto the chute over a specific amount of time is insufficient for most commercial retorting operations. Thus it is desirable to increase the rate at which the material spills over the kiln onto the chute. Numerous devices have been suggested for scrapping the spent shale, ash and fine materials from the top of an upflow bed of oil shale rock. These devices typically comprise various blade arrangements, such as those in U.S. Pat. Nos. 2,980,592; 2,954,328; and 2,895,887 all issued to Deering et al. and 2,954,329 issued to Dhondt et al.
Previously used blade scraping apparatus encountered difficulty in removing the spent shale, ash and fine materials due to the pressure exerted by the high-specific-heat, hydrocarbonaceous recycle gas. The downward flowing gas compacts the upflowing particulate materials, which in turn increases the torque needed to move a scraping apparatus through the upflowing material. As the blades of a scraping apparatus move through the upflow moving material, the material breaks up into smaller and smaller particles until fine materials accumulate at the upper surface in such amounts that a dramatic pressure drop of the downwardly-flowing, high-specific-heat, hydrocarbonaceous recycle gas is observed across this region. This results in less of the gas penetrating downward through the upflow particulate material, which in turn causes a reduction in the efficiency of the retort. Also it has been observed that fine materials compact tighter than larger particulate materials, further raising the torque requirement of the scraping devices. As the torque requirement increases the fine materials production increases, causing a contiuous cycle of fines production and torque increase until eventually the stress tolerance of the apparatus is reached, leading to a shutdown of the retort. Furthermore, the efficiency of the retort is significantly impaired due to the accumulation of fine materials.
It can be seen that there is a need for an apparatus capable of scraping off the upper portion of a particulate bed of solids (e.g., oil shale rock) moving upwardly in a retort while maintaining low torque requirements and minimizing the production of fine materials.